There are many type of backlights used for liquid crystal displays (LCDs). Generally, for full color backlights, the light used to illuminate the backlight has red, green, and blue components. Fluorescent lamps are most commonly used as the light source. With the development of high power LEDs, such LEDs have been replacing the fluorescent lamps in some applications. A combination of red, green, and blue LEDs may be used, or “white light” LEDs may be used. A white light LED uses a blue or UV LED coated with a wavelength-converting phosphor so that the resulting light appears white.
A typical backlight for a small or medium size LCD uses a solid, transparent light guide formed of a polymer. The light source, either a fluorescent bulb or LEDs, is optically coupled to one side edge of the rectangular light guide. The light guide may be in the shape of a wedge or have facets or other types of reflectors that uniformly leak light out of the face of the light guide onto the liquid crystal layers. The red, green, and blue pixel locations of the liquid crystal layer are controlled by electrical signals to effectively act as light shutters for the RGB pixels to create a color image on the LCD screen.
In backlights where only one or a small number of LEDs are used, it is known to form the light guide to have a flattened corner edge, such as a 45 degree angle corner, and mount the LED package in contact with the face of the truncated corner. This is described in U.S. Pat. No. 7,001,058. A suitable LED package for mounting at the corner may be that described in U.S. Pat. No. 6,953,952, which shows an LED package with a window having an area less than 3 mm2. Both patents are incorporated herein by reference. By coupling the LED to the flattened corner, rather than to a relatively long side of the light guide, the light more fully spreads throughout the light guide volume to provide a more uniform illumination of the liquid crystal layers. However, with such corner-coupled LEDs, there is still significant optical coupling inefficiency and nonuniformity, and such backlights have only been suitable for small backlights due to the small coupling area. The nonuniformity is partially a result of a point source of light being applied to a surface of the light guide, rather than a wide homogeneous light source, such as a fluorescent bulb, applied to the surface of the light guide. Further, there are variations in the light emitted by the LED, such as due to color variations across the LED due to uneven deposition of phosphor, or variations in the emission profile of the LED die itself, or varying properties in the LED package. Additionally, if high power LEDs are used, it is difficult to remove heat from the LED without incurring additional area requirements.